Preoxidation of stainless steel for glass-to-metal sealing



April 19, 1960 s. B. BROOKOVER ET AL 2,933,423

PREOXIDATION OF STAINLESS STEEL FOR GLASS--TO-ME1TAL SEALING Filed March3, 1958 INVENTORS 6 5'. 5200/66/42 A TTQQNEIS PREOXIDATION F STAINIESSSTEEL FUR GLASS-TO-IVETAL SEALING George B. Brookover and Elgin M. Tom,Toledo, Ohio, agsisiilgnors to Kimble Glass Company, a corporation ofApplication March 3, 1958, Serial No. 718,566

5 Claims. (Cl. 148-655) The present invention relates to stainless steelalloys having surfaces adapted to metal-to-glass sealing and morespecifically to an improved process of preoxidizing the surfaces ofcertain stainless steel parts as a preliminary measure to creatingimproved durable and vacuumtight metal-to-glass seals, the preoxidizedsurfaces forming the bonding interface therebetween.

As has been known heretofore, certain chromium-iron andchromium-nickel-iron alloys have been particularly well adapted, due totheir specific thermal expansion coefficients, to sealing to aconsiderably wide range of glasses in manufacturing devices forelectronic use. The sealing stresses created in the composite body, madeby sealing a particular metallic part comprised of one of theaforementioned alloys sealed to glass, have been minimized by thermalexpansion coefiicients being properly matched and/ or deviationstherebetween being maintained in proper agreement.

In order to obtain a strong uniform bond between the metal and the glassto which the metal part is sealed, it has been required to furnish arelatively uniform layer of metal oxide over the sealing surfaces of thealloy parts. The oxide layer of alloy preferably possesses properties ofbeing both soluble in the glass at or near the sealing temperatures andfirmly adherent to the base metal of the part. An oxide layer can beobtained on the surface of chromium-bearing alloy parts by exposure to astream of wet hydrogen while the parts are maintained at an elevatedtemperature, but considerable difficulty has been experienced inobtaining the necessary uniformity of the chromium containing oxide filmhaving satisfactory glass sealing properties.

One process for oxidizing chrome-nickel-iron alloys for scaling to glasshas been described in the patent to Kingston, Patent No. 2,502,855,issued April 4, 1950, entitled fPreoxidation of Stainless Steel. Whilethis patent has taught a process of establishing the chromium oxidelayer over the described alloy parts by subjecting the parts to a highvelocity stream of wet hydrogen gas ata constant level of water contentand at an elevated temperature of about 2300 F., objectionable formationof uncontrollably flaky oxide films of varying thickness frequentlyoccurs over the surfaces of the preoxidized parts. Furthermore, theprocess is not considered applicable'to preoxidation of chrome-ironalloy parts, for example, to achieve satisfactory results.

In the mass production of electron discharge tubes such as cathode-raypicture tubes for television reception, for example, it is essentialthat all metal-to-glass seals be as near perfectly vacuum-tight and asmechanically durable as possible inview of employment of the componentparts in relatively high-speed sealing operations. In the production ofglass funnels as one component part of cathoderay picture tubeenvelopes, for example, a lead-in conductor element such as an anodebutton comprised of chromium-iron alloys is normally sealed into thefunnel side wall with interior and exterior surfaces exposed forconduction of an electrical potential from conductivelynite StatesPatent Patented Apr. 19, 1960 coated interior surfaces of the completedtube to external sources. Another example of parts which may be sealedinto such tube envelopes are mounting studs or lugs which are positionedWithin the glass in the skirt or flange portion of the tube face plateextending interiorly for positively retaining an extensivecolor-controlling element such as a shadow mask or line grid in precisealignment with a luminescent target or screen on the tube viewing area.

Obviously, it is exceedingly important that the surface of these metalalloy parts to be sealed to glass have surface layers which do notcontain various oxides which tend to flake away from the base metal. Ascan be readily appreciated such flaking or separation from the basemetal can be a direct cause of air leakage and tube failure due toseparation of the metal surface and the oxide layer of the finishedseal. It has been found that by employment of the subject method, metalalloy parts may be prepared having oxide coatings which facilitate theirsealing to glass in an improved manner, as shown by stripping testswhich measure the strength of resultant i seals.

Also the parts may comprise lead-in conductors such as contact prongswhich are mounted directly into a tube socket. It is necessary that theoxide film on the surfaces of the prongs or anode buttons not adhered toglass be adapted to removal as desired to furnish good electricalcontact between the aforesaid lead-in conductors and socket contacts.Also, the surface oxide must be sufficiently removable from the surfacesof the parts where not sealed to glass to facilitate their welding toother tube elements whereby the part maintains its requisite electricalconductivity.

Accordingly, it is an object of the present invention to provide animproved method of preoxidizing the surface of stainless steel alloyparts under controlled conditions of time, temperature and atmosphere,the oxidized parts adapted to be sealed to electronic glasses in avacuum-tight manner in variegated independent operations.

Another object of this invention is to provide an im proved method ofpreoxidizing chromium-iron alloy parts under controlled conditions toprovide a uniform adherent chromium oxide film over the exteriorsurfaces of the parts to permit their subsequent positive andnon-strippable scaling to glass in hermetic and/or durable seals.

Another object of this invention is to provide a method of preoxidizingthe surfaces of chrome-iron alloy parts into a stable uniform oxide filmhaving properties to permit improved, structurally superiorglass-to-metal sealing of electronic tube envelopes.

Another object of the present invention is to provide the unique methodof preoxidizing chromium steel parts as a preliminary step prior totheir being sealed to glass in vacuum-tight relationship wherein theoxide layer is exceedingly adherent and uniform, the parts beingsubjected to a controlled stream of wet hydrogen gas at both low andelevated temperatures, the degree of water vapor present beingadjustably regulated during the preoxidation.

A still further object of the present invention is to provide a processof preoxidizing chrome-iron alloy shaped parts as a preliminary step totheir being effectively sealed to glass, the preoxidation establishing astable uniform oxide film over the exposed surfaces of the parts bysubjection of the parts to both low and high temperatures whilecontained within a wet hydrogen atmosphere, the water vapor presentbeing controllably regulated from an initial low to high level toachieve optimum expeditious results.

The specific nature of this invention, as Well as other objects andadvantages thereof, will become apparent to those skilled in the artfrom the following detailed (16" of drawings on which by way ofpreferred example only, are illustrated the preferred embodiments ofthis invention.

On the accompanying drawings:

.Fig. 1 is a schematic view of heating and atmosphere controllingapparatus applicable to practicing the .present method of oxidation.

- Fig. 2 is a perspective view of an anode button illustrative of alloyparts suited to oxidation .bytthe prescribed method.

Fig. 3 is an enlarged fragmentary sectional view of a side wall of saidanode button'illustrative of its finally oxidized surfaces.

While the present invention will be described in a preferred embodimentas specially suited to preoxidizing the surfaces of chrome-iron alloyparts .such as anode buttons or positioning studs for glass cathode-raytube envelopes, it is fully understood that the principles of theinvention are equally applicable to preoxidizing surfaces of otherstainless steel parts for other purposes than those specificallydescribed herein.

Referring to the drawings and particularly to Fig. 1, the oxidizingapparatus is comprisedof an elongated tube 11 mounted within a furnace12 and an adjacent cooling chamber 13. The ends of the tube 11 extendbeyond the respective furnace 12 and cooling chamber lfi to provideaccess into the "tube and maintenance of controlled atmosphere therein.Furnace 12has a heating element '14 surrounding a lengthwise portion oftube 11 in spaced relation therefrom and may be fabricated essentiallyof refractory material. Chamber 13 has a fluid coolant such as watersupplied to inlet and outlet lines connected thereto for maintainingminimal temperatures in this area. Chamber 13 is adapted to bothintroduction of the parts to be oxidized and cooling of the same duringlatter stages of oxidation. The 'tube end near chamber 13 is providedwith a hinged access door 15. Tube portion 16 surrounded by chamber 13provides an area where the parts to be oxidized are flushed of theirentrained air prior to oxidation, .and subsequently are cooled afteroxidation.

Tube end 20 is provided with several interconnecting lines 21 and 22adapted to supply both wet and dry hydrogen gas to the oxidizing tube 11from tanks [27 and 28 of hydrogen and nitrogen. Hydrogen tanks 27 andnitrogen tank 28 are provided with a how regulating valves and the gasis conducted through several flow meters 25 and 26 into the tube end 20.The gas stream passing through flow meter'25 is bubbled through a waterreceptacle maintained at a relatively constant temperature of from about18 to 26 C. within bath '24. The gas saturated with the water vapor atthe temperature of the bath is then passed through line 22 into tube end20. The bath temperature maybe varied between the above limits, anintermediate temperature of from '20 to 21 C. being preferred.

An example of parts which may be preoxidized by the subject method ishollow frusto-conical shaped button 18 having. a shape similar to thatillustrated in Fig. 2. The button 18 may be fabricated of an alloy suchas chrome-iron alloy. having compositional designationsas stainlesssteel Product Nos. 430 and 446.

The glass button 18 maybe formed into the shape of a. cup, its largeopen end adapted to extend exteriorly from the side wall of a glass partfor engaging a connecting terminal. The button 13 may be formed of analloy 0 consisting of about 18.0% chromium, 0.5% nickel, 0.5%

apeaaaa titanium, 1 0.40% silica, 0.35% magnesium, and other oxides ofthe elements of'carbon, phosphorus, sulphur, aluminum in minor amounts,the remainder of the alloy or about 80% being substantially iron.expansion coefiicient of this particular type of alloy is approximately117 X 10* cm./cm./degree C. through the range of 20-'600 C.

The thermal 4 Several other alloys which are also applicable tooxidation by the present method are listed below:

Chemical analyses of No. 446 stainless steel alloys:

. ce 114X10- O Coefficient of Thermal Expansion The above-listed alloysmatch the expansion coefficients of the following glass particularlywell for sealing thereto. One example of such glass is one containing58.9% SiO 10.3% PbO; 4.2% A1 0 5.8%"CaO; 2.1% MgO; 1.2% BaO; 7.7% Na 'Oand 9.2% A 0 along with certain other minor constituents in still lesseramounts which are utilized'to further control'glass properties v Eachpart to be sealed into glass is preoxidized in a manner about to 'bedescribed for the purpose o'fforming mechanically strong glass-to metalseals. Areas of the glass parts which are not employed to contact theglass in forming the seal may be freed of the oxide after the requiredglass-to-me'tal seal is effected such as to provide complete electricalcontact with the parts as required.

The method consists of the following:

The chrome-iron alloy parts such as the anode buttons 18, for example,are placed within metal trays 19 which are then slid into the tube 11 inthe introductory or cooled zone 16 of chamber 13. The temperaturemaintained within this area is below 200 F. and preferably is kept about175 F. for the gas stream'to flush away air from the surface of themetal parts.

An atmosphere of wet hydrogen is continually passed through tube 11(from right to leftas shown in Fig. 1)

to sweep the air from the tube. Metal trays 19. containing the parts areretained within zone 16 form interval of from 6 to 8 minutes as aprecautionary measure to eliminate all contaminants such as theoxidizing air fromcontact with the parts. The atmosphere contains bydrogengas containing a controlled amount of water vapor. About '2 to 25%of the hydrogen is saturated with water vapor at a temperature of fromabout 18 to'2.6- C. It maybe preferred to pass -a gas stream containing4 to 10% of the hydrogen saturated with water vapor at a saturatingtemperature of from 20 421 C. The volume of gas which is passed throughtube 11' is dependent upon."

its ,size,-the number of parts being simultaneously oxidized V fromabout 2050 to -2l50 F. The prescribed small quantity of water vapormixed with hydrogen produces a thin coating'of oxide that has anexceptionally firm bond tothe metal. It has been observed that when ofthe hydrogen is saturated with watervapor during this initial stage-ofthe oxidation cycle, the'oxide produced tends to have apoor bond tothe'metal inglassto-rnetal sealing.

The second stage of the oxidation cycle'consists of further subjectingthe parts to a stream of wet hydrogen containing-an increased .amount ofwater-vapor .for approximately 30 or 40 minutes at the statedtemperature of from about 2050 to 2150 F. During this part of the cycle35 to 50% of the hydrogen is saturated with water vapor at a temperatureof 20 to 21 C. The saturation temperature may be varied from about 18 to26 C. as stated, although a temperature of from 20 to 21 C. ispreferred.

This quantity of water vapor present with the hydrogen supplies enoughoxygen to increase the oxide weight or the thickness of the oxide filmsufficiently to prevent burning and an improved joint during subsequentglassto-metal sealing processes. It also permits conducting theoxidation at a faster rate during the secondary stage to allow economicapplication of the method. The weight or thickness of the oxide coatingcan be varied within limits dependent upon sealing requirements bychanging the amount of water vapor in the hydrogen atmosphere during theprimary or secondary stages of the oxidation cycle in order to secure anoxide coating of increased thickness. The greater amounts of hydrogen inthe stated saturation ranges may then be utilized. In this case about to25% of the hydrogen would be saturated with water vapor during theinitial oxidizing stage, while about 50% of the hydrogen would besaturated with water vapor in the secondary oxidizing stage.

The secondary oxidizing period is conducted for approximately 30 to 40minutes and preferably about 35 minutes at the increased water vaporlevel.

At the completion of the second stage of oxidation the metal trays 19are pulled from the furnace section of the tube 11 into a coolingsection 16 of the apparatus. In this area the metal parts are cooled forapproximately 3 to 7 minutes in a stream of wet hydrogen containing theincreased amount of water vapor called for in the secondary oxidizingstage. The temperature within zone 16 at this time is below about 200 F.and preferably is about 175 F. About 35 to 50% of the hydrogen then usedis saturated with water vapor at a temperature of about 20 to 21 C.

After the parts are cooled sufliciently within chamber 13 the trays 19are removed through door 15 at the exit end of the furnace. The furnaceused to practice the present invention may comprise a tubularput-through type with doors on opposite ends or may be modified in manyways to furnish advantages for commercial production. From beginning toend of the method consisting of air expulsion, heating, oxidizing, andcooling steps, a continuous flow of hydrogen gas is maintained Withintube 11. The water or the oxygen content of the gaseous atmosphereduring both the primary and secondary oxidizing cycles is the primaryfactor of the type and character of the oxide which is created on theparts. The inert nitrogen gas may be employed to regulate the rate ofoxidation as required by its addition to the gas stream, serving todilute or increase the amount of effective oxidants in the gasdelivered.

Among the distinct advantages of the new method of oxidizing chrome-ironparts, a much improved oxide coating is obtainable over the metal alloyparts for improved glass-to-metal sealing. The final seals haveindicated much improved characteristics over those previously made byknown preoxidizing methods. Analyses of coatings created on the surfaceof chrome-nickel-iron alloys, for example, have shown that the coatingconsists of a mixture of the oxides of the metals in the original alloy.

While it is possible to vary the elevated oxidizing temperatures Withinthe furnace section to obtain greater or lesser speeds of oxidation,temperatures being varied as much as 100 to 200 from the preferred levelof about 2100 F. have been observed to cause at least some degree ofdegradation of the oxide to create undesirable sealing properties of theoxidized parts. An example of the oxidized film 18a over an anode button18 is illustrated in Fig. 3.

Various modifications may be resorted to Within the spirit and scope ofthe appended claims.

We claim:

1. The method of preoxidizing the surface of chromeiron alloy partsprior to sealing said parts to glass, said method comprising the stepsof exposing the parts to a stream of wet hydrogen for approximately 6 to8 minutes at a temperature below 200 F. to flush away entrained air atthe surface of said parts, subjecting said parts to a stream of wethydrogen for approximately 12 to 18 minutes at a temperature rangingfrom about 2050 to 2150 F. the gaseous stream comprising about 2 to 25%of the hydrogen saturated with water vapor at a temperature from about18 to 26 C., further subjecting said parts to a stream of wet hydrogencontaining an increased amount of water vapor for approximately 30 to 40minutes at the aforesaid temperature ranging from about 2050 to 2150 F.,and cooling said parts for approximately 3 to 7 minutes in a stream ofwet hydrogen containing the said increased amount of Water vapor at atemperature below 200 F.

2. The method in accordance with claim 1, including the step ofsaturating about 35 to 50% of the hydrogen with water vapor at about 18to 26 C. to comprise the increased wet hydrogen stream during secondarysubjection of the parts to a temperature ranging from about 2050 to 2150F.

3. The method of preoxidizing the surface of chromeiron alloy partsprior to sealing said parts into glass in vacuum-tight relationship,said method comprising the steps of exposing the parts to a stream ofwet hydrogen for approximately 7 minutes at a temperature below 200 F.to flush away entrained air at the surface of said parts, subjectingsaid parts to a stream of wet hydrogen for approximately 15 minutes at atemperature of from about 2050 to 2150 F. the gaseous stream comprisingabout 2 to 25% of the hydrogen saturated with water vapor at atemperature from about 18 to 26 C., further subjecting said parts to astream of wet 2 hydrogen containing an appreciably increased amount ofwater vapor for approximately 35 minutes at the aforesaid temperature offrom about 2050 to 2150 F., and cooling said parts for approximately 5minutes in a stream of wet hydrogen containing the said increased amountof water vapor at a temperature below 200 F.

4. The method of preoxidizing the surface of chromeiron alloy parts as apreliminary step prior to thermally sealing such parts to glass inelectron-discharge devices, said method comprising the steps of exposingthe parts to a controlled stream of wet hydrogen for approximately 7minutes at a temperature below 200 F. to flush away entrained air at thesurface of said parts, subjecting said parts to a gaseous stream of wethydrogen for approximately 15 minutes at an elevated temperature ofabout 2100 F., the gaseous stream comprising about 4 to 10% of thehydrogen saturated with water vapor at a temperature of from about 18 to26 C., further subjecting said parts to a gaseous stream of wet hydrogencontaining an increased amount of water vapor for approximately 35minutes at the stated elevated temperature of about 2100 F., the gaseousstream then comprising about 35 to 50% of the hydrogen saturated withwater vapor at a temperature of from about 18 to 26 C., and cooling saidparts for approximately 5 minutes in the stream of wet hydrogencontaining the increased amount of water vapor to a temperature below200 F., the resultant parts having stable oxidized surfaces thereoverapplicable to thermally sealing to glass in vacuum-tight relationship.

5. The method of preoxidizing the surface of chromeiron alloy parts as apreliminary step prior to thermally sealing such parts to glass inelectron-discharge devices, said method comprising the steps of exposingthe parts to a controlled stream of wet hydrogen for approximately 6 to8 minutes at a temperature below 200 F. to flush 7 away entrained air atthe surfacelof. =s'ai'd parts, rsubjecting said parts to a gaseous:stream of 'wet hydrogen for approximately 12 to 18 minutes at anelevated temperature of about .2050 to 2150 R, the :gaseousstreamcomprising about 2 :to 25% :of the hydrogen saturated with water vaporat :a temperature of from about 18 to 726 C., further subjecting said:parts to a gaseous stream :of wet hydrogen containing anincreasedamount of water vapor for approximately 3540 minutes at the statedelevated temperature of about 2050 to 2150 1 the gaseous stream thencomprising about 35-50% of the hydrogen saturated with water vapor at atemperature .of' from about 18to 26 .C., and coolingsaidwparts forapproximately 3 to 7 minutes in the :stream of wet hydrogen containingtheincreased amount of water vapor 1 Uhlig May 25, '1948 Kingston Apr.4, 1950

1. THE METHOD OF PREXIDIZING THE SURFACE OF CHROMEIRON ALLOY PARTS PRIORTO SEALING SAID PARTS TO GLASS, SAID METHOD COMPRISING THE STEPS OFEXPOSING THE PARTS TO A STREAM OF WET HYDROGEN FOR APPROXIMATELY 6 TO 8MINUTES AT A TEMPERATURE BELOW 200*F. TO FLUSH AWAY ENTRAINED AIR AT THESURFACE OF SAID PARTS, SUBJECTING SAID PARTS TO A STREAM OF WET HYDROGENFOR APPROXIMATELY 12 TO 18 MINUTES AT A TEMPERATURE RANGING FROM ABOUT2050* TO 2150*F. THE GASEOUS STREAM COMPRISING ABOUT 2 TO 25% OF THEHYDROGEN SATURATED WITH WATER VAPOR AT A TEMPERATURE FROM ABOUT 18 TO26*C., FURTHER SUBJECTING SAID PARTS TO A STREAM OF WET HYDROGENCONTAINING AN INCREASED AMOUNT OF WATER VAPOR FOR APPROXIMATELY 30 TO 40MINUTES AT THE AFORESAID TEMPERATURE RANGING FROM